1. Field of the Invention
This invention relates in general to a signal processing, and more particularly to method and apparatus for efficient implementation of a multirate LMS filter.
2. Description of Related Art
Rapid changes in the way businesses and government communicate present opportunities for companies involved in broadband network technologies. For example, businesses are driven by a need to add new services constantly. In addition, the convergence of telephone and computer products has demonstrated the need for broadband solutions to support video conferencing, streamed video, telemedicine, and distance learning to name a few.
Digital subscriber line, DSL, technology was first introduced in 1989 as a way to send video and television signals from the telco central office to end-users over standard copper cable. At that time, video-on-demand was perceived as the broadband application that would drive digital subscriber line implementation. The first type of DSL technology was asymmetric digital subscriber line, ADSL, service. As its name suggest, ADSL is asymmetric, i.e., the line has different speeds away from and toward the customer. The focus of DSL has broadened to include applications such as Internet access and work-at-home. Consequently, multiple "flavors" of DSL technology have been proposed.
One proposal is high-bit-rate digital subscriber lines, HDSL. First generation HDSL systems have been in service for a few years. HDSL deployment is substantial in many regions of the world, largely for provisionning of T1 and E1 services to customers, and for digital connection of cellular and PCS base stations to the public telephone network. In North America, HDSL has been used mainly to provide T1 bit rate (1.54 MBPS), full duplex communications between customers' premises and telephone company central office using the regular subscriber loop plant. With HDSL, two pairs of lines are used, each carrying half the payload full duplex, in an arrangement known as dual duplex.
A recent proposal for a high rate digital subscriber loop (HDSL2) included different rates for transmit and receive sampling. This introduces a challenge for efficient echo cancellation. Prior solutions have either assumed symmetric baud rates that are integer multiples of one another (N is transmission rate, M is receive rate, either M/N or N/M is an integer).
In the case of HDSL2 the proposed rates are not integer multiples of one another. Possible solutions involve either a rate change engine at the input to the echo-canceller or interpolating up to a common multiple. Neither one of these solutions is very efficient.
To overcome these limitations, prior art LMS filters first interpolate up to a common multiple of the baud rates, adapt the LMS filter and then decimate the output. The error is then interpolated before being fed back to the LMS. If the baud rates are asymmetric baud rates, e.g., transmitting at 2 MHz and the receiving sample rate is 3 MHz, the signals can be interpolated to a common multiple frequency. Thus, the common multiple frequency rate would be 6 MHz. Then the LMS filter is clocked at that speed, the output is decimated and the errors are cancelled. However, the interpolation and decimation stages dramatically increase the instruction processing rate requirement on the filter since the instruction processing rate, measured in millions of instructions per second (MIPS), increases according to the product of the transmission rate and the receive rate, i.e., N*M.
There are two factors to cause the instruction processing rate to increase. First, in order to cover the appropriate amount of time, twice as many taps are required. However, to span that same time, these taps must be processed in half the time. Accordingly, the instruction processing rate goes up by a factor of four. Thus, in the example where the transmit rate is 2 MHz, and the receive rate is 3 MHz, the instruction processing rate requirement goes up by a factor of four to 24 MIPS.
It can be seen then that there is a need for an efficient LMS filter implementation that prevents the instruction processing rate requirement from increasing.
It can also be seen that there is a need for an efficient LMS filter implementation wherein the interpolation and decimation are performed in the LMS filter element at the same time so the instruction processing rate requirement stays very low.